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Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
How would higher voltage be less current in a half-bridge?
Assume you push a certain number of Joules in a Projectile. Lets say... E=10J. Those 10J is Force*AccelerationLength. To pass the AccelerationLength given a mean speed v gives you a travel time (t). Basically i just describe: you do need a certain time to push the 10J. This time is constant as long as you push 10J and have equal initial velocity. Lets say the time is t=1ms. The 10J within 1ms equates to a Power. (Energy/Time = Power, J/s = W) So your coilgun has a mechanical power of E/t=10kW. Given your efficiency is.. lets say.. 10% inevitably you need an electrical power of 100kW. Those 100kW can be achieved by 100V, 1kA... or 500V and 200A. since Power = Voltage*Current. So the higher the voltage the less current.
You want the highest possible current because that gives you the highest magnetic field.
Well if you put it that way there is no need for a halfebridge design. If you have absolutely no efficiency optimization in mind, then go ahead Realistically, above a certain current your efficiency will drop so far.. its almost below 0. A certain coil shape with a certain projectile can only push a certain amount of energy with a certain efficiency. These are physical limits... You are building a motor here. The higher the power, the bigger the engine! Since your engine will on someday have a fixed size, you will have someday a fixed power. This power will fed by P=U*I.
To get the highest possible current, you need more voltage.
Yes+No=Yo? In a coil the current is: dI = U*dt / L. - So yes. You can maximize U. - You could however also maximize dt. Ok. dt is fixed, due to the limitations above. (1ms as representative number) - You can also minimize L.
a) A lower L is done by less turns. Since L ~ n^2 you can take away half the turns, you get 1/4 the Inductance. 1/4 the inductance will yield 4x the initial current given a constant U 4x the initial current yields double the force with half the turns. (4x current * 0.5x turns = 2x Force).
b) Lets do the same with doubling U. Double U gives you the double peak current. The number of turns stay the same. 2x Current * 1x Turns = 2x Force.
Both version [a) and b)] have incrased your force. You want however the Force to be constant, because in the end you have constant power.
To counteract the higher force in a) you will turn down the voltage (so the current does not rise as high) To counteract the higher force in b) you will increase the number of turns, to get slower current rise in the given time
Remeber: this are only the basic thoughts for a Halfbridge. A SCR-Design is inherently different - inherently crappy .
Registered Member #58215
Joined: Wed Dec 30 2015, 11:27AM
Location: Boise, Idaho
Posts: 65
Thanks for the information, it is helping me simulate and brainstorm a lot. I guess I pick wire and IGBTs I can afford at this stage, and then design according to them. Solve for the voltage I am at and number of turns to give me my peak current that corresponds to the IGBTs, and then use that. I'll have to expand my knowledge as I go as IGBT half-bridges are a lot more advanced than the simple switches I thought they were. There is a point where you can't just hakve the number of turns and get more force- this because you have to keep your current at a level the IGBTs can handle. In a design going purely for Kinetic Energy, then, It would be desirable to have a higher voltage to get the same current, but with more turns. The downside of course is a longer rise time (and longer shut off time after the switch is flipped, I think?) There must be an "optimal" point for the given materials where it is better to just ad anither coil, a point decided by your design constraints. Putting together a materials list for actual ordering of parts, and then I can design accordingly to the parts I have. That will help me learn faster than theorizing. I desoldered the sketchy setup my capacitors had, and want to mount them to something more securely. I am thinking of taking a copper pipe or other cheap length of copper and cutting and straightening it into small bars to drill holes in and solder the caps too. Anyone have a better/safer idea?
Edit 1/22/16 Got permission from the professors to work on the project. It took a senior deciding to work on a 10kV can crusher for his senior project for them to okay me for 350V. Also got permission to use all lab materials including the oscilloscopes- regarding measurements with a scope during a discharge, does anyone have any advice on how to safely set up the measurement? The scopes we use have earth ground as a reference, and I'm afraid of shorting the circuit hooking it up to the wrong place. Thinking I may hook up the ground to the negative lead of my charging circuit, and the tip to the location I want to measure (immediately before the inductor). I should be able to capture the waveform of the discharge like this, but if anyone has advice... I would love to not break a scope. Our scopes on the 10x setting (probe only goes to 10x) are rated for 600V and 10Mhz I believe.
Edit 3/29/16 Project isn't dead, just got put on hold. I got mono and all the associated problems and have been focusing on school. Next up I am ordering diodes and switches.
Registered Member #58215
Joined: Wed Dec 30 2015, 11:27AM
Location: Boise, Idaho
Posts: 65
Went to an electronics surplus store and while they didn't have a lot of things I was hoping to find, I did pick up this for fun:
Thinking I'll connect a few disposable camera charging circuits to it and make a single-stage gun just to have fun with. It doesn't really fit with my actual longer term coilgun project but it'll be fun and show off a basic gun. I can also use it as a testbed for spin stabilization and a few other things.
With regards to my actual project: Did more simulation work. I have a few ideas for a more non-conventional coilgun. I believe that ready access to large capacitors and (as DerAlbi has discussed in his thread) SCRs are the two largest reasons for hobbyist coilguns bwing so inefficient.
I'm simulating one design that makes use of batteries and another that uses a large number of much smaller capacitors. I think with the battery one I can reach speeds in excess of 200 m/s with 30% efficiency for a 10mmx30mm iron projectile cylinder (10mm diameter). Those estimates are from paper math and FEMM simulation. I'll have to do a lot of testing before I can finalize the design for a prototype but mostly the cost of the whole system is a few hundred dollars out of my budget. I'm graduating in a few months, though, so hopefully I'll have more money then for the project.
I'm still going to design and simulate things in the meantime, so I'll get started and ask here with regards to my first issue:
I need to know the position of the projectile +-.5 mm. Anyone have ideas for this? I was thinking a laser rangefinder placed behind the projectile but the systems I have found affordable are not so accurate. I have also considered capacative sensors on the inside of the barrel because they can be made very thin, or just small loops of wire placed inside the barrel/between coils and some interpolation.
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
I think with the battery one I can reach speeds in excess of 200 m/s with 30% efficiency for a 10mmx30mm iron projectile cylinder (10mm diameter)
You are talking about an 18g projectile at 200m/s == 375J projectile energy. are you sure you have not forgotten any decimal point in the 200m/s.. also be aware of that the current involved in the coils increased with the square of the velocity (for a battery design) and with the third power in a capacitor design (if you pous constant energy per stage).
I need to know the position of the projectile +-.5 mm. Anyone have ideas for this? I was thinking a laser rangefinder placed behind the projectile but the systems I have found affordable are not so accurate. I have also considered capacitive sensors on the inside of the barrel because they can be made very thin, or just small loops of wire placed inside the barrel/between coils and some interpolation.
I tried building a range finder circuit what would have an appropiate sampling rate. It did fail mainly due to missing optics. Also bending the barrel and reflections inside the barrel are hard to deal with. Capacitive sensors can be made and i would be interested in their design. However i would think that evaluating capacitive sensors is quite complex and you cannot reuse the same circuit for multiple capacitors (at least its complicated). I use the phase shift of high frequency resonant inductive sensors since this can be cascaded easily but the circuitry is a bitch. I would say the most feasible option is the light trap.
Registered Member #58215
Joined: Wed Dec 30 2015, 11:27AM
Location: Boise, Idaho
Posts: 65
the 30% efficiency is my guess from the work... i do not think it will be super accurate without recuperating energy.
not sure what you mean in your second paragraph reminder about coil current and velocity
A laser-based sensor may not work for me because of what I want to do with the shaping and spinning of the projectile. I would not need to chain capacitors but I could potentially by analyzing an AC signal running through the chain. i think inductive sensors might be best because I can use extremely thin wires for the sensors.
I'll be relying heavily on PWM and rapid changes from a microcontroller already, but with a few MHz clock speed I think things will execute well enough.
I won't be buying hardly anything until I am done designing. I need to purchase a lot of my own equipment for when I cannot use the school's too. Deralbi do you have your own scope?
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
The 30%eff is reachable, this wasnt an issue for me. I just thought about my 100m/s estimates are already reaching 800A at 300V while you speak of 200m/s meaning 2.4kA and your voltage is even less, since you talk about a battery driven design so you current will be even higher. I dont see how you will manage this kind of current in a halfbridge. I know you projectile is smaller than mine.. however the ballpark figures still apply since this is close to an order of magnitude too crazy to be reasonable. I am not sure but if your models or math predict that you can reach 200m/s with a 18g projectile there is something fundamentally wrong. Can you elaborate how you get those figures?
Registered Member #58215
Joined: Wed Dec 30 2015, 11:27AM
Location: Boise, Idaho
Posts: 65
A higher number of smaller stages carefully controlled so that the maximum force possible (center of coil in most designs) is always as close to the projectile as possible
and then a lot of stages. Big capacitors are terrible for efficiency because you end up dumping most of the energy before the projectile is in the optimum position, and then a big C messes with the time constant so a lot of energy isnt available for the small window that is optimal for maximizing the force on the projectile. then your projectile hits halfway into the coil and coasts until your system fires the next coil. this means that A. half of every coilgun barrel is wasted as far as energy going into yhe projectile is concerned B. most energy is dumped at a poor time
I'm still not sure at all what you mean with 800A and such... I do not plan on exceeding 400A but again, more stages. You are making a portable gun as a primary requirement, I only wish that it be able to be carried and set up by one person. without caps it gets lighter.
and of course: Simulating my design in FEMM and supplementing with paper and having things actually work are very, very different. When i get back home (in over a week) from christmas I can send you what i actually did
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
I think what DerAlbi means is this: The lower your current, the wider your pulse width, the wider your pulse width, the less energy you can put in the right place, timewise and obviously spacewise.
Registered Member #58215
Joined: Wed Dec 30 2015, 11:27AM
Location: Boise, Idaho
Posts: 65
Found this pretty cool video. I hope he shows off more of his design. He can hit 90 m/s with this one which seems to be running off of relays or something
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Project Log- Coilgun testing
Realistically, above a certain current your efficiency will drop so far.. its almost below 0. 
A certain coil shape with a certain projectile can only push a certain amount of energy with a certain efficiency. These are physical limits... You are building a motor here. The higher the power, the bigger the engine! Since your engine will on someday have a fixed size, you will have someday a fixed power. This power will fed by P=U*I. 
.
